Effect of short-term exposure to elevated temperatures and light levels on photosynthesis of different host–symbiont combinations in the Aiptasia pallida/ Symbiodinium symbiosis

The physiology exhibited by symbioses between dinoflagellates (zooxanthellae) and hosts such as reef corals may be dictated by the host, the symbiont, or the synergistic effect of both partners. We compared the oxygen fluxes of two laboratory populations of the symbiotic sea anemone Aiptasia pallida, originally collected from Bermuda and Florida. A. pallida from Bermuda contained clade B zooxanthellae and A. pallida from Florida hosted clade A zooxanthellae. Both freshly isolated zooxanthellae and intact anemones were compared as a function of light intensity at culture (258C) and elevated (328C, 348C) temperatures. Zooxanthellae isolated from the Florida anemones had higher net photosynthetic rates at 328C (Pmax: 6.2 6 0.9 mmol O2 h21 mg chlorophyll a [Chl a]21 vs. 0.7 6 3.6 for Bermuda algae), with the Bermuda algae exhibiting pronounced photoinhibition at higher irradiances. At 348C, the clade A (Florida) symbionts had lower net photosynthesis than at 328C (Pmax: 3.6 6 2.2 mmol O2 h21 mg Chl a21), but the Bermuda symbionts had high rates of net O2 consumption at all irradiances. The intact Bermuda anemones, however, exhibited much less pronounced effects at elevated temperatures than did the isolated Bermuda zooxanthellae. Bermuda anemones containing the Florida symbionts had higher net oxygen fluxes than those with the Bermuda symbionts at elevated temperatures and irradiances. This study demonstrates that the physiology of the intact symbiosis is dictated by both partners and that studies of the intact symbiosis may not fully reveal the thermal liability of the algae. Symbioses between cnidarians (e.g., corals, octocorals, sea anemones) and dinoflagellates (zooxanthellae) are crucial for the operation of coral reef ecosystems. Metabolic exchange between the partners is pivotal, with the zooxanthellae providing photosynthetic energy to the host while the host provides protection and nitrogenous wastes (Muscatine and Porter 1977). Much recent attention has been given to the diversity of these symbionts and their distribution (Baker 2003; Goulet and Coffroth 2004; Lajeunesse et al. 2004). There has been some consideration of how this diversity affects photosynthetic characteristics (reviewed in Lesser 2004). There is, however, little information about whether symbionts associated with the same host species display wide or narrow physiological ranges. For example, particular zooxanthella and host genotypes may have wide tolerances, allowing the association to exist over a wide latitudinal range or a large depth range on a reef. On the other hand, a host 1 Corresponding author ([email protected]).